This invention relates to friction materials such as those used in brakes and clutches of motor vehicles such as trucks and automobiles. More particularly, the invention provides to a friction material using iron (steel) powder for use in disk and drum brakes.
Generally, friction materials for brakes contain asbestos serving as a strengthening material, ceramic (inorganic) powders such as of oxides and carbides serving as friction coefficient (hereinafter referred to as .mu.) increasing agents, and graphite and soft metal serving as friction adjusting agents, these ingredients being bound by an organic binder such as, for example, a resin. However, the use of asbestos and also some inorganic powders have been criticized because of the risks associated with their manufacture and use. Thus, it has been proposed to replace asbestos by steel fiber as a strengthening material and some or all of the ceramic (inorganic) powder by iron powder as a friction coefficient increasing agent, it being noted that a sintered alloy type friction material has been used in some two-wheeled vehicles.
For example, Japanese Patent Application Laid-Open No. 157673/1980 states that " . . . the conventional means of adding ceramic powder in order to increase friction coefficient .mu. is not desirable since it grinds a mating material . . . " and proposes a friction material " . . . which has increased .mu. by using high carbon iron powder while using steel fiber or inorganic fiber as a strengthening material . . . " while Japanese Patent Application Laid-Open No. 11373/1982 proposes a friction material using steel fiber and atomized iron powder. U.S. Pat. No. 3,835,118 proposes a friction material using sponge iron powder and steel fiber (including the case of using no steel fiber), and British Patent No. 1,206,554 proposes a friction material using short fiber powder-like steel wool and white cast iron powder. Further, British Patent No. 2,073,782A proposes that a sintered alloy using iron powder be used as a friction material. Thus, a friction material which uses iron powder or a combination of iron powder and steel wool has been known. In this connection, it is to be noted that since iron powder is generally less expensive than steel fiber, it is desirable that in a friction material which uses both, the amount of iron powder be increased while the amount of steel fiber is decreased.
It is necessary for friction materials to have high .mu.. To increase .mu., it is known to incorporate in a binder an organic material similar in nature to rubber to thereby increase the actual area of contact with a mating member such as a brake. Using such a binder, however, the heat resistance of the friction material does not increase; rather, .mu. is decreased when the friction material is used under high temperature conditions particularly at high speeds.
Another way to increase .mu. is to incorporate in a friction material a harder material than a mating member so as to expect a scratching force to be produced on the mating member during braking. This means, however, is not desirable since it increases wear on the mating member and forms a remote cause of creaks produced during braking.